Rapid Shift

Biggest glacier in East Antarctica is melting from above and below

By Chris MooneyDecember 16, cross-posted from the Washington Post online

Scientists at institutions in the United States and Australia published in December 2016 a set of unprecedented ocean observations near the largest glacier of the largest ice sheet in the world: Totten glacier, East Antarctica. And the result was a troubling confirmation of what scientists already feared — Totten is melting from below.

The measurements, sampling ocean temperatures in seas over a kilometer (0.62 miles) deep in some places right at the edge of Totten glacier’s floating ice shelf, affirmed that warm ocean water is flowing in towards the glacier at the rate of 220,000 cubic meters per second.

These waters, the paper asserts, are causing the ice shelf to lose between 63 and 80 billion tons of its mass to the ocean per year, and to lose about 10 meters (32 feet) of thickness annually, a reduction that has been previously noted based on satellite measurements.

This matters because more of East Antarctica flows out towards the sea through the Totten glacier region than for any other glacier in the entirety of the East Antarctic ice sheet. Its entire “catchment,” or the region of ice that slowly flows outward through Totten glacier and its ice shelf, is larger than California. If all of this ice were to end up in the ocean somehow, seas would raise by about 11.5 feet.

“This ice shelf is thinning, and it’s thinning because the ocean is delivering warm water to the ice shelf, just like in West Antarctica,” said Don Blankenship, a glaciologist at the University of Texas at Austin and one of the study’s co-authors. Blankenship was not on the research vessel, but he and his colleagues helped the Australia-based researchers with understanding the contours of the seafloor so they could plan their field investigations into where warm and deep waters could penetrate.

The lead author of the research, published Friday in Science Advances, was Stephen Rintoul, a researcher with the University of Tasmania in Hobart and Australia’s Commonwealth Scientific and Industrial Research Organization, or CSIRO. Totten glacier is, more or less, due south of Australia and relatively close to one of Australia’s bases of operations on the ice continent, Casey Station.

Rintoul and his colleagues, on board the government vessel Aurora Australis, were able to navigate extremely close to the Totten ice shelf edge in January of 2015, when an opening in the sea ice allowed the ship to get in closer than one ever has before. This is how they were able to gather the required ocean observations — and to detect the warm water.

A Conductivity-Temperature-Depth profiler being deployed near the front of the Totten Glacier. (Credit: Steve Rintoul CSIRO and ACE CRC)

The researchers took ocean measurements at 10 separate points along the floating Totten ice shelf. And at two of the stations, they found that the ocean underneath was extremely deep. There was a six-mile-wide canyon at a depth of 600 meters (nearly 2000 feet) that then branched into two narrower canyons, each reaching greater depths. One of them was over 800 meters deep (more than 2,500 feet) the other was 1,097 meters deep (3,600 feet). Each was about one to two miles wide.

It was in these deep undersea canyons, and a few shallower areas as well, that warm ocean water, called modified circumpolar deep water, was flowing inward powerfully towards Totten glacier. And the previously measured loss of ice from the ice shelf matched closely with the amount of heat that the ocean was delivering, the paper found.

Granted, calling the waters reaching Totten at great depths “warm” is a bit of a misnomer —they are slightly below the freezing point. However, at the extreme pressures and depths involved, the freezing point of ice itself lowers, making these waters more than warm enough to melt ice.

Measuring the warm water reaching Totten was, until now, a missing puzzle piece in determining what’s happening with the glacier. Prior research, for instance, had shown the presence of cavities that warm water could enter, and scientists believed this was occurring because they had observed Totten thinning and lowering in the water. But as NASA glaciologist Eric Rignot put it to the Post at the time, “it is one thing to find potential pathways for warm water to intrude the cavity, it is another to show that this is actually happening.”

Now, scientists are showing that it’s actually happening.

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The researchers are interested in Totten not only because of the massive global consequences were it to be destabilized, but also because it could help solve a riddle from the Earth’s past. Researchers have calculated that during previous warm eras, such as during the Pliocene, about 3 million years ago, global temperatures not too much higher than those that exist today led to radical amounts of sea level rise. It’s too much of an ocean surge for the loss of West Antarctica, alone, to explain — so they’ve been going looking to East Antarctica to close the sea-level budget from those eras.

And it turns out that like West Antarctica, East Antarctica features several regions — including Totten — where massive amounts of ice rise above the ocean level, but are grounded deep below it. In the case of Totten glacier, its so-called “grounding line,” which is where the glacier begins to lift off the seafloor and to float, forming an ice shelf with an ocean cavity beneath it, is nearly a mile and a half deep.

Granted, none of this means that Totten is contributing much to sea-level rise — yet. The large loss of ice from the ice shelf doesn’t raise seas because that ice is already afloat. But the weakening of the ice shelf is troubling because the shelf holds back Totten’s more dangerous ice, and when it goes it will allow that ice to flow more easily into the ocean.

For Blankenship, the new study, combined with past aircraft and satellite research on Totten, puts the remaining piece in place and suggests an increasingly clear picture of ocean-driven melt that could lead to growing instability.

“The whole process is here and going on,” he says. “This is the biggest potential contributor in East Antarctica. It needs to be understood.